Hydrofoils and How They Work

Hydrofoils and How They Work

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What is a hydrofoil? A hydrofoil is a watercraft that is supported on ski-like pontoons while in motion, with the bulk of the hull remaining entirely above the water (Encarta Encyclopedia 2002). Hydrofoils were first seen about in 1869. Emmanuel Denis Farcot was issued a patent on a boat that he had developed to go faster through the water because of less resistance. If you look at his design, he was using many little foils along the side of his boat to lift it out of the water in order to reduce the drag on the hull of the boat.

Hydrofoils are not only used on boats. Guidoni and Croco took the idea of hydrofoils and put then on seaplanes. Their main reason for puting foils on the plane was to make the landings smoother and to make takeoffs much quicker. The plane can get out of the water much faster with foils because there is less drag inhibiting the plane from gaining speed to get airborn. This is useful when taking off of short lakes or in rough water.

There are many purposes for hydrofoils. The main reason people or buisnesses install hydrofoils onto their watercraft is to decrease the drag of the boat as it travels through the water. By decreasing the drag, the watercraft is able to travel at higher speeds while burning less fuel. This makes use of the watercraft much more economical and provides a smoother, more comfortable ride because the watercraft rides above the reach of most of the waves.

Another reason to use hydrofoils is because they are fun. The picture below shows a water ski that has been transformed into something that you sit on with your feet in front of you while you are "floating on air" behind a ski boat.

When used on sail boats, hydrofoils can increase the top speed by quite a bit. Once the hull of the sail boat is out the the water the drag from the water is much less. Simply explained, there is less surface area "rubbing" on the water to slow the boat down. This means that even with gentle winds, a small boat can really get moving.

Most hydrofoils lift the watercraft that they are supporting in the same way that airplane wings keep the plane supported in the air. With enough lift on the water foils, the hull of the watercraft is lifted out of the water.

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Foils create lift when the water traveling over the top surface of the foil goes faster than the water traveling over the bottom (by curving the top of the foil). Where there is faster velocity (of the water in these cases) there is lower pressure. If there is higher pressure on the bottom of the foil, there will be a lifting force on the foil. Most foils are positioned from parallel to the hull to 5 degrees up. They are angled from side to side to allow a portion of the foil to come out of the water as the boat rises, thus allowing the boat to maintain stability by keeping a portion of the foils in submerged in the water.

However, foils do not have to be curved on the top. Some foils are simply positioned to have a positive angle of attack. These foils act by simply pushing the water down at high speeds and creating a lifting force on the watercraft by action and reaction. These foils are actually slightly less stable than the previous foils because they do not stay partially submerged in the water. Watercrafts with these kinds of foils tend to "skid" when turning at high speeds because there is no part of the craft in the water to provide the necessary friction for turning.

The main reason that hydrofoils work is because of Newton's Third Law: for every action force there is an equal and opposite reaction force. In layman's terms, this means that for every unit of force that the hydrofoil is pushing down on the water, the water is pushing back with the same force. This resistive force is what supports the weight of the watercraft on the hydrofoil.

These diagrams show how the pressure is distributed as described by Bernoulli's equation(Po = P1 + ½rv1² + rgy1 = P2 + ½rv2² + rgy2).

Po Stagnation Pressure [Pa] or [lbf/ft2]
P Pressure [Pa] or [lbf/ft2]
r Density [kg/m3] or [lbf/ft3]
V Velocity [m/s] or [ft/s]
g Gravitational Constant [m/s2] or [ft/s2]
y Height [m] or [ft]

His equation, simply worded, says that when an object is moving through a fluid, the faster the fluild moves over the surface, the less the pressure on the surface. This means that if the fluid (water in this case) moves faster over the top of the foil then there is more pressure on the bottom than on the top.

Another equation that applies to the lift produced by hydrofoils is Euler's Equation (d(p+rgy)/dn = rv²/R).

P Pressure [Pa] or [lbf/ft2]
r Density [kg/m3] or [lbf/ft3]
V Velocity [m/s] or [ft/s]
g Gravitational Constant [m/s2] or [ft/s2]
y Height [m] or [ft]
n Vector in Radial Direction ---
R Radius of Curvature of Streamline [m] or [ft]

This equation says that as you go further from the center of the radius of curvature of a streamline, the pressure on the streamlines increases. The upper surface of the foil is closer to the center of curvature of the streamlines, therefore there will be a lower pressure than the ambient pressure above the foil. The difference between the pressure on the top surface and the ambient pressure at the bottom surface will produce a net pressure that will cause the lift (Tina Rosado).

Hydrofoils are beneficial in many areas, but their usage is not applicable everywhere. One place you would not want to even attempt using hydrofoils, unless you are an extremely experienced operator, is on a river. Hydrofoils need a certain amount of water to be able to support the boat. If the water is not deep enough the foils will hit bottom and most likely cause damage to themselves, the boat, and possibly the passengers.

Another limitation that comes to mind is similar to a limitation of airfoils, namely they will stall when placed at too great an angle of attack. This could be a problem, but fortunately hydrofoils do not have to be angled to such an extreme degree in order to produce the desired lift. Since the density of the water is greater than that of air, the necessary angle of attack for a hydrofoil is minimal compared to what an airfoil would need. The definition of angle of attack is shown in the diagram below.

Another limitation of hydrofoils is the foil coming out of the water. If a wave is large enough to hit and lift on the original hull of the boat the hydrofoils will be pulled down away from the boat with extreme force. This could cause the foils to be ripped from the boat or cause other forms of damage. Also if the boat is moving fast enough to make the foils come out of the water (e.g. on the back side of a wave) the stability of the foils is lost and the boat may tip over or even be sucked back down to the water surface when the foils re-enter the water. This would happen if the front foil became airborn causing the bow to drop. The bow dropping would cause the new angle of attack to be negative, thus pulling the boat down rather than pushing it up. Another problem is ventilation. This is where air gets under the foil. Since air is almost 1000 times less dense than air, the boat will fall until it is supported by the water again. These are possible problems, but they can be almost completely avoided with good engineering design.

The uses for hydrofoils are really only limited to your imagination. Hydrofoils can be used on boats, on waterskis, on planes, on just your feet (AYRS 22), and anything else that you could imagine. Hydrofoils are used to increase maximum speed of watercraft and to decrease the drag of the watercraft through the water. They are used to help water planes take off in shorter distances and to give tourists a smoother ride accross the English Channel. There really are no limitations to what can be built with foils as long as physics is obeyed.

Works Cited
* AYRS. Sailing Hydrofoils. England: Amateur Yacht Research Society, 1970.
* Blackburn, Grahmam. The Illustrated Encyclopedia of Ships Boats Vessels and other Water-Borne Craft. Woodstock: The Overlook Press, 1978.
* Burghardt, David. Introduction to Engineering Design and Problem Solving. New York: McGraw-Hill, 1999.
* Hayward, Leslie, "The History of Hydrofoils," Hovering Craft and Hydrofoils, Vol 4. No. 8 (May 1965) through Vol. 6, No. 6 (Feb 1967).
* Internet Source: Sky Ski, http://www.skyski.com/.
* Internet Source: International Hydrofoil Society, http://www.foils.org/
* Internet Source: Pictures by Onnig Cavouk, http://www.seaflights.com/.
* Internet Source: Rosado, Tina. "Hydrofoils". http://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html
* Internet Source: http://www.wingo.com/dakh/.
* Navarra, John Gabriel. Superboats. New York: Doubleday & Company, 1997.
* Serway, Raymond A. Physics For Scientists and Engineers. Fifth Edition. United States: Brooks/Cole, 2000.
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